This invention relates to dynamic output impedance adjustment.
A driver is a digital electronic circuit for holding a binary value, and communicating it to other circuits to which it is connected. The binary value is represented by a voltage level. It is common to connect a driver to a data bus for communicating the binary value to a receiving circuit by xe2x80x9cdrivingxe2x80x9d the bus to a desired voltage level. In a typical parallel interface, one of the voltage levels (HIGH or LOW) is a power rail voltage, and the other voltage signal is a xe2x80x9cswing voltagexe2x80x9d away from the power rail voltage. That is, the difference between a voltage HIGH signal on bus line and a voltage LOW signal is called the xe2x80x9csignal swing.xe2x80x9d
The driver has an inherent output impedance. The driver""s output impedance when driving the bus to a voltage HIGH level may differ from the driver""s output impedance when driving the bus to a voltage LOW level. In addition, the bus has an inherent transmission line or characteristic impedance, and the receiving end of a parallel-terminated system has an input impedance, called the termination impedance. To obtain a high rate of data transfer on the bus, the characteristic impedance should closely match the termination impedance.